1. Field of Endeavor
The present invention relates to optics and more particularly to the reduction of damage in optics.
2. State of Technology
A typical means of specifying the laser power handling capability of an optic is to make a determination of its laser damage threshold. Such a threshold is defined as the level of power, and more specifically the fluence, at which any modification of the optic surface or bulk occurs as a result of the passage of the laser pulse. Scientific study of the mechanism and manifestations of laser damage are ongoing. For example, in U.S. Pat. No. 3,999,865, issued Dec. 28, 1976, Milam, et al. describes a system for determining the mechanism responsible for laser-induced damage in a sample which utilizes a procedure of sequentially irradiating a large number of damage sites using a tightly focused laser beam whose intensity is constant in time. A statistical analysis of survival times then yields a determination that damage was due to one of the following mechanisms: (1) linear absorption, (2) nonlinear absorption, (3) absorbing inclusions, (4) mechanical defects, or (5) electron-avalanche breakdown. However, a method for reducing the initiation of damage sites or mitigating the growth of existing damage sites is not provided.
There are many avenues that can be used to increase the laser power handling capability of an optic. These methods include, but are not limited to: (1) fabrication process improvements, (2) use of alternate materials, and (3) post-fabrication treatment or mitigation techniques. Investigations on material properties and process improvements are numerous and many are within the purview of vendors of optical components. In U.S. Pat. No. 6,099,389, issued Aug. 8, 2000, Nichols, et al. describe a method for improving the grinding and polishing of optical parts that minimizes damage during the operation of high power lasers in the ultraviolet range. In U.S. Pat. No. 5,325,230, issued Jun. 28, 1994, Yamagata, et al. describe a method for making optical components from a special composition of high-purity synthetic silica that are able to withstand prolonged exposure to high-power ultraviolet light. Fabrication process improvements, such as those disclosed in U.S. Pat. Nos. 6,099,389 and 5,325,230, relate to semi-finished products that have not been finally polished. The present invention relates to a combined fabrication and post-fabrication treatment technique that significantly increases the lifetime of fused silica optics, such as lenses and windows, for use with high-power ultraviolet laser beams.
Several researchers have previously reported that the damage thresholds of some optical materials for a particular laser could be increased by first illuminating the optical materials with sub-threshold fluences of the same laser. In U.S. Pat. No. 5,472,748, issued Dec. 5, 1995, Wolfe, et al. describe a method for permanently increasing the laser damage threshold of multilayer dielectric coatings by irradiating with a sub-damaging fluence at the operational wavelength of 1060 nm. However, it is not apparent that such a procedure would work for the fused silica component particularly at the ultraviolet wavelength of 355 nm. Similarly, in U.S. Pat. No. 4,667,101, issued May 19, 1987, Siekhaus describes an apparatus for identifying and locating weak spots that could potentially lead to laser-induced damage by subjecting the material to laser intensities that are less than the intensity actually required to produce the damage, and notes that these weak spots may be eliminated by sustained exposure to the laser beam. Siekhaus also notes the possible use of the apparatus for cleansing the optical surface of impurities that could potentially lead to damage. However, because there are many possible mechanisms that lead to damage, as discussed by Milan, et al. in U.S. Pat. No. 3,999,865, it is not apparent that the apparatus described by Siekhaus in U.S. Pat. No. 4,667,101 will be able to significantly reduce the initiation of damage sites when a finished fused silica optic is exposed to a high-power ultraviolet laser.
The method of increasing laser damage threshold by irradiating with a sub-damaging fluence is not a new phenomenon and is commonly known as laser conditioning. The laser wavelength, material identity and material disposition play important roles in determining whether laser conditioning occurs and the degree to which it occurs.
Virtually all references in the literature dealing with laser conditioning consider the conditioning phenomenon using the 1064 nm laser wavelength. This wavelength is in the infrared region of the electromagnetic spectrum, whereas, in our invention, the wavelength of interest is in the ultraviolet portion. The effect of wavelength is a very important parameter in determining whether there can be a conditioning effect. In fact, there is evidence to show that a laser conditioning effect using shorter wavelengths would not be seen. Arenberg and Mordaunt [“Experimental Investigation of the Role of Wavelength in the Laser Conditioning Effect”, Nat. Inst. Stand. & Tech. (US.) Spec. Pub. 756, October, 1987 pp. 516-518] had reported that laser conditioning has been observed for an optic for 1064 nm wavelength exposure but that an increase in the laser damage threshold at the shorter 532 nm wavelength had not been observed. One would conclude from this fact that decreasing the wavelength to produce the conditioning effect would not be efficacious.
In U.S. Pat. No. 6,205,818, Mar. 27, 2001, Seward describes a method of rendering fused silica resistant to compaction caused by ultraviolet laser beam irradiation. The method makes the clear distinction between two types of laser damage—those associated with absorption and those associated with compaction. The concern in U.S. Pat. No. 6,205,818 is focussed upon the latter form of damage, with specific concern about the birefringence of the silica optic and the transmitted wavefront alterations produced after use at wavelengths shorter than the one contemplated in the present invention. In the present invention, laser damage concerns are entirely dominated by the former type of laser damage, namely absorption. Catastrophic damage is presumed to be associated with absorbing centers or multiphoton effects associated with defective regions, not with compacted regions. Furthermore, the specific remedy called out in U.S. Pat. No. 6,205,818 suggests an exposure to a laser beam with a fluence higher than the one anticipated under normal operating circumstances. One cannot apply this type of remedy in our case because to do so would quickly result in catastrophic damage to the optic. The treatment methodology in the present invention provides for sub-damage threshold illumination with an ultraviolet laser beam, up to the level of the operating fluence. In the detailed description of the present invention, this method will be shown to be very effective based on the data accompanying the invention description.
The material to be conditioned plays a role in determining whether laser conditioning occurs and the degree to which it occurs. Virtually all of the references in the literature involving laser conditioning focus attention on the treatment of dielectric multilayers, as in U.S. Pat. No. 5,472,748, issued Dec. 5, 1995, by Wolfe, et al. Such multilayers involve stacks of silica and other crystalline materials with a higher index of refraction, such as hafnia, zirconia, titania and the like. Laser damage in these multilayers has been correlated with nodule defects, stemming chiefly from the deposition parameters associated with the high index component. It would be natural to assume then that laser conditioning to improve the laser damage performance of these multilayers would be somehow associated with these nodule defects. However, in bulk fused silica, these defects are not present. Therefore, the main mechanism for laser conditioning cannot exist and it then becomes difficult to imply in an obvious manner that laser conditioning, in any form, can take place for bulk fused silica. Runkel, et al. [“Laser conditioning study of KDP on the Optical Sciences Laser using large area beams”, Proc. SPIE-Int. Soc. Opt. Eng. (1998), 3244; Laser-Induced Damage in Optical Materials: 1997, 51-63] discusses laser conditioning for bulk materials, but the reference is only for crystalline potassium dihydrogen phosphate (KDP) crystals and not amorphous fused silica. It remains non-obvious that laser conditioning for bulk fused silica is a viable method of ameliorating laser damage issues in that material.
All previous studies of laser conditioning deal with its effect on the laser damage threshold. A functional or practical measure of the laser power handling capability of an optic should take into consideration the severity of the laser damage and its scope in terms of the concentration of damage sites on the optic surface. For example, small damage sites that do not grow and are limited to a very small fraction of the optic surface would be a damaged optic in the classic, absolute sense but may be acceptable for use in a functional or practical sense. The presentation, “Catastrophic failure of contaminated fused silica optics at 355 nm” by Genin, et al., presented at 2nd Annual International Conference on Solid-State Lasers for Applications to Inertial Confinement Fusion, Paris, France, Oct. 22-25, 1996 describes a study of failure of contaminated fused silica optics at 355 nm.
Methods for reducing the initiation of damage sites that lead to catastrophic failure of the fused silica optics are of critical importance to the high-energy fusion laser community. They are also of critical importance to the multi-billion dollar semiconductor capital equipment market. In the lithography equipment for manufacturing of silicon chips, ultraviolet light in the range of 340-360 nm is primarily used. However, manufacturers would like to use shorter wavelengths such as 193 nm and 248 nm. These wavelengths are becoming common in biomedical devices as well. All of these wavelengths are produced by a series of ultraviolet lasers and images through fused silica optics. Unfortunately, no suitable methods have been shown to significantly reduce the initiation of damage sites when fused silica optics are exposed to high-power lasers at these wavelengths.